1. Field of the Invention
This invention relates to a rotary magnetic head device which is used in conjunction with a magnetic recording and/or reproducing apparatus, especially a small-sized magnetic recording and/or reproducing apparatus, such as a digital audio tape recorder (DAT) or video tape recording (VTR).
2. Description of the Prior Art
It is known in the prior art to use a rotary magnetic head device of the so-called upper rotative drum type in a magnetic recording and/or reproducing apparatus, such as a digital audio tape recorder (DAT) or a video tape recorder (VTR).
Such upper rotative drum type rotary magnetic head devices of the prior art are shown in FIGS. 4, 5 and 6 to be comprised of a lower drum 52, 62 or 72, respectively, which is secured to a mounting base plate, not shown, provided in the main body of the device, and which carries a fixed core 51, 61 or 71 of a rotary transformer 50, 60 or 70. A rotary shaft 54, 64 or 74 is mounted for rotation coaxially of the lower drum 52, 62 or 72 by a bearing 53, 63 or 73, such as, a ball bearing, and an upper drum 55, 65 or 75 is fixedly attached to the rotary shaft 54, 64 or 74. A magnetic head 56, 66 or 76 is attached to the underside of the upper drum 55, 65 or 75 facing toward the lower drum 52, 62 or 72. A rotary core 58, 68 or 78 of the rotary transformer 50, 60 or 70 is secured to a flange 57, 67 or 77 attached in turn to the upper drum 55, 65 or 75 and has its face opposing a face of the fixed core 51, 61 or 71 of the rotary transformer 50, 60 or 70. The upper and the lower drums combine to provide a tape guide drum on the outer periphery of which the magnetic tape is wrapped and adapted to travel therepast with a prescribed wrapping angle. The upper drum 55, 65 or 75 is rotatably driven by an electric motor, not shown, adapted for driving the rotary shaft 54, 64 or 74 so that the tape contact surface at the radially outermost part of the magnetic head 56, 66 or 76 will be brought into sliding contact with the tape surface for recording and/or reproducing prescribed information signals.
Recently, magnetic recording and/or reproducing apparatus which is so small in size as to be portable has been proposed. In reducing the size of the apparatus, it is essential to reduce the size of the rotary magnetic head device incorporated into the main body of the apparatus. For reducing the outer diameter and axial extent of the rotary magnetic head device, it is required to simplify the structure while reducing the number of the components of the device.
However, when the drum diameter is to be reduced in the rotary magnetic head device having the above described structure according to the prior art, a problem arises in respect to suitably accommodating the rotary transformer in the drum, especially in view of the bearing device provided for rotatably supporting the shaft carrying the rotary upper drum, and which needs to be of at least a predetermined radial thickness about the rotary shaft for accommodating friction reducing elements, such as, bearing balls.
Thus, in the prior-art arrangement shown in FIG. 4 wherein the core type rotary transformer 50 is disposed radially outwardly of the bearing device 53 and the cores 51 and 58 face each other at planar confronting surfaces, if the outer diameter of the drums 52 and 55 is reduced, a sufficient area of such confronting surfaces of the rotary transformer 50 cannot be provided since the bearing device 53 is required to have a predetermined radial dimension, as stated above. The result is that signal transmission characteristics of the rotary transformer 50 are significantly lowered.
In the prior-art arrangement shown in FIG. 5 wherein a cylindrical rotary transformer 60 is arranged outside of the bearing device 63 for assuring that the confronting cylindrical surfaces of cores 61 and 68 of the rotary transformer 60 can be of a sufficient area, it is impossible to achieve a sufficient thickness of the fixed core 61 or of the rotary core 68 of the rotary transformer 60 when reducing the outer drum diameter so that difficulties in the assembling operation are increased with a significantly lowered operational efficiency.
In order to overcome the aforementioned problems inherent in the above described rotary magnetic head device in which the rotary transformers 50 and 60 are arranged radially outside of the bearing devices 53 and 63, as shown in FIGS. 4 and 5, it has also been proposed to provide an arrangement in which the rotary transformer 70 having its cores 71 and 78 with planar confronting surfaces is disposed axially above the bearing device 73, as shown in FIG. 6.
When the rotary transformer 70 is disposed above the bearing device 73 in this manner, the overall axial thickness or height of the device is increased in reverse proportion to the reduction in the drum diameter so that it becomes impossible to achieve a rotary magnetic head device of both small axial height and small outer diameter.
It will be noted that, in the above described conventional upper rotative drum type rotary magnetic head devices the upper drums 55, 65 and 75 are provided with wall sections 55a, 65a and 75a extending upwardly from radial walls and serving as the guide surfaces for the magnetic tape, so that they are of a predetermined thickness in the heightwise or axial direction, such wall sections 55a, 65a and 75a define, a dead space opening upwardly in these upper drums 55, 65 and 75 for smoothing the rotation of the upper drums.